Diagonal Mirror

There is a diagonal mirror in the TUB used to direct light
into the science instrument, guide camera, both or neither.
The four positions for the diagonal mirror can be selected
from the sdiagmir_fe program. sdiagmir_fe is started by clicking
on its icon on either the gouda or karnak computers.

Position 1 puts the diagonal mirror out of the light path,
sending all light into the science instrument.

Position 2 puts a full surface flat mirror into the beam, which
diverts all the light to the guide camera for field acquisition; no
light reaches the detector. A white card on the back of the mirror in
position 2 directs light from the TUB quartz or line lamps into the
instrument.

In position 3, a portion of the diagonal mirror that has a hole
in it is positioned so that light from the object passes through the
hole to the detector, and the surrounding mirror is available for
offset guiding.

In position 4, the mirror is entirely out of the way, but a
periscope is in position to view the slit (if the Kast Spectrograph is
mounted). The Kast slit jaws are tilted
slightly, and light reflected from them is directed to the periscope,
which passes the light on to the guide camera for guiding.

The standard diagonal mirror is shown at right, with a
total field of view of approx. 330" x 240", which the guide camera
(with a field of view of about 120") can access by moving it on an x-y
stage. The hole for light to go to the instrument is about 2
arcminutes in diameter, thus limiting the field for direct imaging
using the mirror in position 3. Other diagonal mirrors are available
which have larger holes, giving a wider field of view at the
Cassegrain focus, but limiting the area in which to locate off-axis
guide stars. For example, the Gemini IR camera, uses a
different diagonal mirror so that a field of view of 3 arcminutes is
possible.